Aerogels and xerogels are extremely porous materials representing the lower end of the density spectrum of man-made materials. Densities as low as 0.003 g/cm.sup.3 have been reported for silica aerogels [L. W. Hrubesh, Chemistry and Industry, 24 (1990) 824]. These fascinating materials have numerous unique properties as a result of their ultra-fine microstructure. Aerogels with the porosities in the range of 0.85 to 0.98 may be transparent and translucent. The porosity is defined as the fraction of the sample volume which is pores. Aerogels exhibit strong Rayleigh scattering in blue region and very weak in red region. In the infrared (IF) region of electromagnetic spectrum, the radiation is strongly attenuated by absorption leading to application in radioactive diode systems, which effectively transmit solar radiation but prevent thermal IR leakage [J. Fricke and R. Caps in Ultrastructure Processing of Advanced Ceramics: ed. J. D. Mackenzie and D. R. Ulrich (Wiley, New York, 1988)]. Unusual acoustic properties (sound velocity as low as 100 m/s) suggests use in impedance matching and other acoustic applications. Aerogels with a refractive index of 1.015 to 1.06 cover the region not occupied by any gas or solid resulting in applications in high energy physics.
Silica aerogels with their thermal conductivities as low as 0.02 W/mK (0.01 W/mK when evacuated) find application in superinsulation systems [J. Fricke in Sol-Gel Technology for Thin Films, Fibers, Performs, Electronics and Speciality Shapes: Ed. L. C. Klein (Noyes Publications, Park Ridge N.J., 1988)]. Fricke has described several aerogel applications based on their insulating properties. They include reduction of heat losses through windows, energy-effective greenhouses, translucent insulation for solar collectors, and solar ponds for long-term energy storage. Fricke has also discussed the mechanism for thermal transport through aerogel tiles and the efforts of density and gas pressure on their thermal conductivity.
Since aerogels are made by sol-gel processing, their microstructure may be tailored to optimize properties desired for specific applications. Various precursors, including metal alkoxides, colloidal suspensions, and a combination of both under several mechanisms of gelation may be used to synthesize gels [C. J. Brinker and G. W. Scherer, Sol-Gel Science (Academic Press, San Diego, 1990)]. By varying aging conditions such as time, temperature, pH, and pore fluid, the parent wet gel microstructure may be altered [P. J. Davis, C. J. Brinker and D. M. Smith, Journal of Non-Crystalline Solids; P. J. Davis, C. J. Brinker, D. M. Smith and R. A. Assink, Journal of Non-Crystalline Solids; R. Deshpande, D. W. Hua, C. J. Brinker, and D. M. Smith, Journal of Non-Crystalline Solids]. In addition to metal oxide gels such as silica, aerogels may be made from wet precursor gels which contain both inorganic and organic components or from organic gels. For the composite gels, the organic and inorganic phases may be mixed on different length scales such that the organic component resides solely on the internal pore surface, is incorporated into the spanning gel structure, or forms a separate (from the inorganic phase) gel structure.
The conversion of the wet gel to a dried aerogel or xerogel requires the removal of large quantities of solvent. As solvent is removed, the gel tends to shrink as a result of capillary pressure. The capillary pressure generated during drying is related to the pore fluid surface tension, y.sub.LV, and contact angle, .theta., between the fluid meniscus and pore wall as follows, EQU P.sub.c =-(2y.sub.LV cos.theta.)/r (1)
where r is the pore radius. For submicron capillaries, such as in silica gels, very large stresses are developed during drying. Aerogel synthesis involves the reduction of capillary pressure by lowering the surface tension, y.sub.LV, to avoid shrinkage of the wet gel during drying.
Aerogels are not new materials. They were first reported by Kistler almost 60 years ago [S. S. Kistler, Nature, 127 (1931) 741]. However, recent advances in sol-gel processing technology along with increasing environmental concern have regenerated interest in energy conservation and alternate thermal insulation applications. The most common method of making aerogels involves directly removing the pore fluid above its critical point (for ethanol T.sub.c =243.degree. C., P.sub.c =63 bar). The critical point is a chemical specific point on the pressure-temperature phase diagram and at temperature and pressure above the critical point values, a liquid and gas cannot coexist but rather, a supercritical fluid will. This avoids liquid-vapor menisci and thus, capillary stresses during drying and essentially preserves the wet gel structure. An alternate low temperature method involves replacing the pore fluid with liquid CO.sub.2 and then removing CO.sub.2 above its critical point (T.sub.c =31.degree. C., P.sub.c =73 bar). All of these prior approaches require high pressure supercritical processing to reduce shrinkage.
Although aerogels exhibit very unique properties, they suffer from several drawbacks for commercial applications. An important disadvantage is their high processing cost because of the need for high pressures associated with supercritical drying of monoliths which require large autoclaves. For the high temperature process significant chemical and physical changes in gel structure can occur as a result of the greatly accelerated rates of aging and changes in the equilibrium behavior of various reactions whereas the low temperature carbon dioxide exchange process is limited to certain pore fluids, since they must be miscible in liquid CO.sub.2.